The present invention relates to the field of methods and devices for evaluation of the eyeball. More specifically, in one embodiment the invention provides an improved method and device for measurement of pressure in the eyeball, otherwise known as a tonometer which is especially safe and easy to use, enabling home use in some embodiments.
Measurement of the pressure in the eyeball is necessary in connection with the diagnosis, evaluation, and control of glaucoma. Glaucoma is the group of ocular diseases having in common an elevation or instability (diurnal variation) in intraocular pressure beyond the tolerance of the eyeball. In many patients, glaucoma is controlled by medication, and regular monitoring of intraocular pressure can be beneficial in management of these patients.
Instruments for measurement of the intraocular pressure are known as tonometers. There are two commonly used types, those which contact the eyeball and those which do not. Both types are used by a professional and require an office visit by the patient. In the aplanation contact tonometer, a flat-ended probe is placed in contact with the cornea or sclera. Then the probe is pressed against the eyeball to produce a flattened area of specified diameter. The applied force is measured, and the pressure is calculated. In the non-contact tonometer a puff of air is directed onto the surface of the eyeball, and the deflection of the surface is measured optically. Both of these methods are applicable to a clinical setting and require a trained professional to take the measurement. Frequent self monitoring of intraocular pressure by glaucoma patients could be useful for early detection of a sudden pressure rise.
A vibration tonometer has been proposed. According to one such device, the resonant frequency of a vibrating mass placed in contact with the eyeball is used. The eyeball surface provides the spring-restoring force for the vibration, and thus determines the resonant frequency of the system. The intraocular pressure determines the stiffness of the "spring." Unfortunately, since the eyelid is very soft and compressible, the measurement must be made with the probe end in direct contact with the eyeball so that the eyelid compressibility will not substantially impact the measurement. As a result this "vibration tonometer" is generally a contact type instrument, and would typically be used in a professional setting.
Another type of vibration tonometer (which also requires contact with the cornea) utilizes a forced vibration of 20 Hz frequency and 0.01 mm amplitude which is applied to the cornea, and the counter pressure is measured by a piezoelectric crystal. According to a variation of this method, the probe, which touches the eyeball, is driven by a dynamic (moving coil) system such as used in loud-speakers, and a piezoelectric pressure transducer measures the reaction pressure of the eyeball. According to other variations, the vibrating mass is replaced with a piezoelectric quartz disc coupled to a quartz membrane placed in contact with the cornea. Such systems suffer from a number of restrictions. For example, accuracy of the device would be significantly impacted if the device is not used in direct contact with the eyeball.
A vibration-based tonometer has also been described in which a vibrator is placed on the eyelid near its edge, and is driven at ultrasonic frequencies around 25-30 kHz. The waves produced by such a device are on the order of the resonance frequency for compressive body waves in the eyeball, since compressive waves travel at about 3.times.10.sup.5 cm/s in water, and the distance from the front to the rear of the eyeball is 2.4 to 3 cm. The vibration is detected by a light beam which is directed on the cornea and reflects onto a photo detector, and the intraocular pressure is intended to be determined from the amplitude of the vibrations induced in the eyeball. Again, amplitude of the waves will be impacted by the eyelid, thereby impacting the accuracy of the device.
It has also been proposed to measure intraocular pressure by the speed with which mechanical oscillations spread across the surface of the eyeball. The eyelid is kept closed, and transmitting and receiving transducers are placed some distance apart on the eyelid above the cornea. The intraocular pressure is determined by the elapsed time of arrival of surface waves started at a specific time. The transmitter is triggered and a wave train is switched on. On reaching the receiver the wave is displayed on an oscilloscope triggered along with the transmitter. Unfortunately, a wave train which is suddenly switched on will generally contain a range of frequencies. Furthermore, surface wave propagation is dispersive (the velocity depends on frequency). As a consequence, the constituent waves will arrive with different delays, and when they are recombined to form the received wave train, it will not have a sharp onset. As a result the intraocular pressure will not be accurately determined.
For example, if a frequency of 5 kHz is employed, the wavelength of the waves is about .lambda..apprxeq.4 cm. Therefore, from v=.nu..lambda. (velocity=frequency.multidot.wavelength) and T=S/v (time =distance/velocity), the calculated transit time T.apprxeq.10 msec for the largest feasible transmitter-receiver separation S.apprxeq.2 cm, and there will be .about.50 waves on the cornea between the transmitter and receiver. Solving for eyeball pressure using this information and differentiating, it is found that .DELTA.p/p=-3 .DELTA.T/T (from equation 2, below). Therefore if the intraocular pressure is to be measured to 10% accuracy, the arrival time would need to be measured to within 3.2%, i.e., 320 .mu.sec. For this to be possible, the rise time of the signal at the receiver should be less than 320 .mu.sec. This in turn requires the amplitude modulation of the 5 kHz signal to include sidebands up to .about..+-.1500 Hz, as can be shown by Fourier analysis. So long as .lambda.&lt;5 cm, v=.lambda..nu.=(.pi.rp/.lambda.).sup.1/2 (where v is wave velocity of propagation, r is eyeball radius, p is eyeball pressure, and .nu. is wave frequency) so velocities would be spread out about (5000.+-.1500 Hz).sup.1/3 /(5000 Hz).sup.1/3 =(1.+-.0.3).sup.1/3, i.e., about .+-.9%. As a result, the transit times of the various Fourier components will also be spread out by the same factor or a total of about 1800 .mu.sec. This is far larger than the allowable range of 320 .mu.sec. In fact, with the parameters given above, the accuracy of pressure measurement by this method would be expected to be relatively low, such as below about 30%, as may be seen by substitution in the above calculations.
To achieve a 10% accuracy, the frequency would have to be increased above 25 kHz. However, this is on the order of the resonance frequency for compressive body waves which travel from the front to the rear of the eyeball. The compression waves reflected from the rear of the eyeball would reach the receiver very quickly, and thus, resonant compression vibrations of the eyeball would give spurious readings. It is clear that measurement of elapsed time until arrival of surface waves started at a specific time is not an appropriate method for accurate determination of intraocular pressure.
From the above it is seen that an improved tonometer and method of measuring pressure of the eyeball is needed.